Crushed Oyster Shell

Crushed oyster shell is the slow option. A cuttlebone can be scraped hollow inside a few weeks in a busy tub, whereas a scoop of shell worked into the substrate gives itself up over months, which suits an enclosure you would rather not keep opening. At least 95 per cent of it is calcium carbonate, laid down mainly as calcite, the harder and less soluble of the two crystal forms molluscs build with, alongside trace magnesium and sodium. Hardness is the whole point: it is what stops it vanishing in a fortnight.

Everything with a shell or a shed skin needs a supply, and the demand is not evenly spread. Millipedes calcify more heavily than almost any other land arthropod and add fresh segments at every moult, so what they need keeps climbing as they grow; most species eat their own cast skin afterwards to get some of it back. Land snails are the extreme case, since the shell is close to being nothing but this mineral, and a snail running short ends up with thin, pitted, easily broken growth. Isopods sit between the two, recovering much of the old cuticle before they turn and making up the shortfall by eating. Hermit crabs will take it from a dish.

You may have read that oyster shell beats crushed eggshell because eggshell attracts mites. We went looking for evidence of that and could not find any, in the literature or in anything the hobby cites, so it is not a reason to choose between them. The mites people have in mind are mould and grain mites, and what brings those in is damp plus something mouldering. If mites are the problem, the things to look at are ventilation and how long uneaten food sits out, which our guide to mites and pests in isopod enclosures goes through.

Offering it

How you do it depends on what you keep. For millipedes, work it into the substrate at build time so they meet it as they burrow, and scatter a little near where you feed. For isopods, tip a small pile onto bare substrate or into the food dish, and top it up every few weeks as it goes. Snails and hermit crabs do best with a shallow dish they can return to. You need very little, and overdoing it slightly will not foul anything.

Supplied crushed, with nothing added. Stored closed and dry it keeps indefinitely. Worth having alongside a cuttlebone, which is soft enough for smaller and younger animals to make use of straight away, and Malawi light limestone if you want a permanent block sitting in the enclosure as hardscape. The reasoning behind all three is in our isopod calcium guide.